(a) Field of the Invention
The present invention relates to a method and device for detecting a liquid level in a container and, more particularly, to a liquid level indicator and a method for detecting a liquid level within an opaque container.
(b) Description of the Related Art
Liquid containers made of opaque materials such as a metal are widely used for retaining therein a liquid. These containers do not explicitly show the amount of the liquid remainder within the container for the user. The amount of the liquid remainder within the container can be generally detected by simply lifting the liquid container to estimate the weight thereof or by shaking the container together with the liquid for estimation, if the container is light in weight as is the case of a spray can or juice can. If the liquid container cannot be easily lifted, however, as is the case of a heavy container, a transparent level gauge or sight glass is generally used for detecting the liquid level within the container.
Recently, vending machines are increasingly used wherein a concentrate of a soft drink and the like is diluted with water and sold to a customer at the request thereof. A liquid container retaining therein a concentrate soft drink is stocked in the vending machines for each of the soft drinks. The stock container is replaced by a new container filled with the concentrate soft drink after the stock container is found empty. The replaced container is then cleaned, disinfected, again filled with the concentrate, sealed with a pressured gas and delivered to another vending machine as a new stock container.
It is generally difficult to lift or shake the stock container located within the vending machine to estimate the amount of the liquid remainder within the container due to a relatively large weight of the container and a narrow space inside the vending machine. A transparent level gauge, if installed on the stock container, raises the fabrication cost thereof and complicates the outer shape of the container to thereby hinder the washing and disinfection operation. A sight glass for the container also complicates a gas-seal mechanism for the container to thereby raise the fabrication cost of the container.
In short, a transparent gauge or sight glass for each of the containers is not practical for the liquid container in the vending machine because of the added fabrication cost therefor.
Another method may be considered for detecting the liquid remainder by attaching a thermo-sensitive color tape (thermo-tape), which is affected by a temperature thereof to change the color thereon, to the outer wall of the container at a suitable height or by printing a color mark onto the outer wall of the container with a thermo-sensitive color ink (thermo-ink) having a similar temperature characteristic. In this method, the thermo-tape or thermo-ink (simply referred to as thermo-tape hereinafter) is first heated (or cooled) to rise (or fall) locally in temperature above (or below) the ambient temperature and then observed to change its color while it is being naturally cooled or warmed toward the ambient temperature.
During the natural fall (rise) in the temperature of the thermo-tape, the lower portion of the thermo-tape facing the liquid inside the container falls (rises) slowly to the ambient temperature whereas the upper portion of the thermo-tape facing the gas inside the container falls (rises) rapidly. Accordingly, the thermo-tape exhibits different colors at an instant during falling (rising) from the initial temperature to the ambient temperature. Thus, the liquid level within the container is estimated by detecting the boundary between the different colors. This method takes advantage of the difference in the thermal conductivity or thermal resistivity between the liquid and gas.
In the method using the thermal conductivity for detecting the liquid level as described above, cooling for the thermo-tape below the ambient temperature may be implemented by blowing cooled air, which is not practical however, because of the difficulty or high cost in implementing the device for cooling the air. When a refrigerator is used for cooling the thermo-tape instead, the thermo-tape must be detached and attached, which results in the reduction of the adhesiveness to render the thermo-tape useless.
Examples of the methods for heating the thermo-tape above the ambient temperature include blowing heated air, pressing the thermo-tape by using an external heater such as an iron, embedding a heater within the thermo-tape, touching the thermo-tape by hand etc., which methods are more practical than the method for blowing cooled air or cooling the thermo-tape.
The methods for heating the thermo-tape as described above, however, involve respective drawbacks therein. The method using heating by hand only provides a limited temperature rise which scarcely allows sufficient discrimination of temperatures by observation of the colors. The method using heated air dose not allow uniform heating of the thermo-tape. The method of using an iron or installing an electric heater within the thermo-tape requires a temperature control device for attaining an accurate and uniform temperature by, for example, fitting the thermo-tape with a temperature sensor, thereby raising the fabrication cost for the liquid container. The electric heater also requires an outlet for a power source in the vicinity of the liquid container, or otherwise, requires long and troublesome electric leads which may hinder the handling of the container or other work.
The external electric heater such as an iron, if used, covers the thermo-tape during the heating, which requires removing the iron from the surface of the thermo-tape during observation of the color thereon. In this case, removal of the iron results in a rapid natural cooling of the thermo-tape, which may hinder the effective detection of the color change of the thermo-tape by the operator. Further, possible overheating may damage the nature of the thermo-tape, which requires a thermo-fuse to further raise the fabrication cost of the container.